D. R. Dykaar

Generation of high-power sub-single-cycle 500-fs electromagnetic pulses.

We have generated sub-single-cycle pulses of electromagnetic radiation with pulse energies as high as 0.8 μJ and pulse lengths < 500 fs. The 10-dB width of the spectrum is 1.5 THz. The transmitter is a GaAs wafer illuminated at normal incidence by 120-fs, 770-nm pulses from a Ti:sapphire chirped-pulse amplifier system while a pulsed electric field is applied across the surface. The pulse energy of the far-infrared radiation is found to be a quadratic function of bias field and a nonmonotonic function of laser intensity.

Terahertz emission from electric field singularities in biased semiconductors

Summary form only given. Fast electrical transients and terahertz (THz) radiation are generated when biased coplanar striplines are excited within the gap by short laser pulses. This generation process is particularly efficient when the excitation is carried out close to the anode and has been observed in a number of material systems. In this work we go one step further and use electric field singularities in metal-semiconductor microstructures to generate efficient THz radiation and electrical transients.

LIMITED THICKNESS EPITAXY IN GAAS MOLECULAR BEAM EPITAXY NEAR 200 C

The low‐temperature limit to GaAs molecular beam epitaxy (MBE) is studied at temperatures from 250 °C to room temperature. Using transmission electron microscopy of layers grown under a variety of conditions we show that, as for Si MBE, there is an epitaxial thickness hepi at which a growing epitaxial layer becomes amorphous. The temperature, growth rate, composition, and defect density all appear to be constant during growth of the epitaxial layer, and (in analogy with Si MBE) we tentatively associate the breakdown of epitaxy at hepi with roughening of the growth surface. We demonstrate that hepi depends strongly on composition, increasing rapidly with Ga/As ratio at fixed temperature. At fixed Ga/As ratio, hepi shows an abrupt increase from <200 to ≳5000 A at 210 °C. The results have implications for the growth of GaAs/GaAs for high‐speed photodetectors, as well as possible applications to GaAs/Si heteroepitaxy.

Interferometric characterization of 160 fs far‐infrared light pulses

We report the first interferometric characterization of freely propagating, subpicosecond, far‐infrared (FIR) light pulses. FIR light was generated via short pulse photoexcitation of a semi‐insulating InP wafer. The half width of the intensity interferogram was 230 fs. The FIR light contained frequency components from 3 to 150 cm−1.

Far-infrared light generation at semiconductor surfaces and its spectroscopic applications

Femtosecond pulses of far-infrared (FIR) radiation can be generated in the depletion field near semiconductor surfaces under optical excitation via the inverse Franz-Keldysh effect or electric-field-induced optical rectification. This coherent mechanism differs from previously proposed incoherent mechanisms and is expected to dominate for nonresonant excitation as well as for resonant excitation with optical fields. Two femtosecond time-resolved spectroscopic applications using a visible excitation pulse and a FIR probe pulse are described. >

Sticky pulses: two-color cross-mode-locked femtosecond operation of a single Ti:sapphire laser.

We demonstrate mode-locked two-wavelength synchronized operation of a single cw-pumped Ti:sapphire laser. There is no active feedback or external cavity. The measured cross correlation is 105 fs. Cross mode locking is maintained for relative cavity length detunings as large as ±3 μm and wavelength offsets as large as 20 nm.

Picosecond pump and probe spectroscopy utilizing freely propagating terahertz radiation.

We demonstrate the feasibility of scaling up a terahertz-pulse generation scheme for use with a 10-Hz amplified femtosecond laser system. Visible pulsed excitation combined with a far-infrared probe should prove to be a powerful picosecond time-resolved technique.

Electro‐optic sampling and carrier dynamics at zero propagation distance

We demonstrate 200 fs (full width at half maximum) electrical pulse generation and detection using low‐temperature grown GaAs and total internal reflection electro‐optic sampling. System limited electrical rise times (10%–90%) of 150 fs are measured. Carrier dynamics are investigated for short times in GaAs and InP using this technique and compared with transient reflectivity measurements.

High-speed coplanar transmission lines

The authors present measurements of picosecond pulse propagation on coplanar strip transmission lines for which speed (i.e., group velocity), as well as phase and amplitude information are measured. Electrode effects are studied using transmission line loops 1 mm in diameter with cumulative propagation distances as long as 6 cm. The intrinsically low dielectric constant of coplanar-air transmission lines is shown to result in high signal speed and low attenuation. The authors use the data to verify the applicability of a model based largely on empirical formulae for radiation loss (geometry) and conductor absorption. The model is then used to identify optimal design criteria (for a given bandwidth) and is extended to the case of small dimensions. >

Timing-jitter stabilization of a colliding-pulse mode-locked laser by active control of the cavity length

Colliding-pulse mode-locked lasers are an excellent source of ultrashort pulses but exhibit timing jitter that can seriously degrade the temporal resolution in experiments that require synchronization to an external source. We describe a method that synchronizes the colliding-pulse mode-locked laser to a high-stability external oscillator by actively controlling the cavity length of the laser and reduces the absolute timing jitter from ~25 ps rms to only 1.7 ps over the 1-kHz bandwidth, which contains most of the laser phase noise. By using this method, the repetition rate of the laser pulses is held constant at 100 MHz.